| The shortage of water resources and pollution have become an important problem to be solved all over the world.As a scientific and effective water treatment process,membrane bioreactor(MBR)is widely used in the field of wastewater treatment and water resources regeneration.However,the membrane pollution caused by MBR operation will greatly reduce the membrane flux and increase the operation cost,which limits the further popularization and development of MBR process.Periodic chemical cleaning with sodium hypochlorite(NaCIO)is essential to restore the membrane permeability in a membrane bioreactor(MBR).However,the chlorination of membrane foulants results in the formation of disinfection by-products(DBPs),which will cause the deterioration of the MBR effluent and increase the antibiotic resistance in bacteria in the MBR tank.In this study,the formation of 14 DBPs during chemical cleaning of fouled MBR membrane modules was investigated.Together with the effects of biofilm extracellular polymeric substances(EPS),influences of reaction time,NaCIO dosage,initial pH,and cleaning temperature on the DBP formation were investigated.Then based on the important role of extracellular polymers(EPS)in the structure and composition of biofilms,the effect of EPS on the formation of DBPs in chemical cleaning of NaClO membranes and the mechanism of action were analyzed.Finally,based on the above research results,from the perspectives of optimizing membrane cleaning process conditions and regulating the physiological characteristics of biofilm bacteria on the membrane surface,the control method of DBPs in the process of MBR membrane NaClO chemical cleaning was studied.The results of the study show that among the DBPs generated after 24 hours of exposure to NaCIO cleaning solution,haloacetic acid(HAAs)is the most generated disinfection by-product,and the concentration can often reach 3000 μg/L,accounting for more than 80%of the total DBPs,of which TCAA(Trichloroacetic acid)dominates.The concentration of THMs,mainly trichloromethane(TCM),can often reach hundreds of μg/L,while the relatively unstable haloacetonitriles(HANs),halogenated acetones(HKs)and The concentration of trichloronitromethane(TCNM)under neutral conditions is less than 70μg/L,and even under alkaline conditions,it is less than the detection limit.Haloacetic acids(HAAs)and trichloromethane(TCM),composed over 90%of the DBPs,were increasingly accumulated as the NaCIO cleaning time extended.By increasing the chlorine dosage,temperature,and pH,the yield of TCM and dichloroacetic acid(DCAA)was increased by up to a factor of 1-14,whereas the yields of haloacetonitriles(HANs)and haloketones(HKs)were decreased.Either decreasing in the chlorine dosage and cleaning temperature or adjusting the pH of cleaning reagents toward acidic or alkaline could effectively reduce the toxic risks caused by DBPs.When NaCIO is combined with acid and alkali for cleaning,the mixing of NaClO and citric acid as a cleaning agent produces the most DBPs,and the mixing of NaCIO and NaOH as a cleaning agent produces the least DBPs.After the EPS extraction pretreatment of the contaminants on the MBR membrane surface,due to the destruction of the biofilm structure,the formation of DBPs within 12 hours of the initial cleaning was faster than that of the samples without EPS extraction.At 6 hours,the former produced more THMs than the latter 30%.Chlorination of endogenous organic matter(IOM)and extracellular polymer(EPS)extracted from sludge bacteria found that the former has five times the potential of producing TCM than EPS,even when EPS is completely unable to produce HANs by-products.Both IOM can react with NaCIO to produce large amounts of trichloroacetonitrile(TCAN)and dichloroacetonitrile(DCAN),and the toxicity risk value of the resulting DBPs is nearly a hundred times greater than the latter.There is a critical concentration of sodium hypochlorite to kill bacteria in sludge,and sodium hypochlorite below the critical concentration can not make bacteria burst and release IOM in a short time.Confocal laser scanning microscopy(CLSM)images showed that EPS,particularly polysaccharides,were highly resistant to chlorine and might be able to protect the cells exposed to chlorination.Moreover,the plane distribution of the biofilm is uneven,and some areas with special structure have strong resistance to chlorination,which can also remain on the surface of the membrane after a long time of chemical cleaning.The results showed that the potential of TCM formation of IOM with 1mg TOC was 181 μg/L,which was five times as much as that of EPS,even when EPS could not produce Hans like by-products,IOM can react with NaCIO to produce large amounts of Trichloroacetonitrile(TCAN)and dichloroacetonitrile(DCAN).The results of toxicity risk analysis showed that the total toxicity risk value of DBPs chlorinated by IOM was nearly 100 times higher than that of EPS.There is a critical concentration of sodium hypochlorite to kill the bacteria in sludge.Under the critical concentration,a short time contact with sodium hypochlorite can not break the bacteria and release IOM.Under the condition of free chlorine concentration/MLSS=1,the structure and activity of bacteria in the sludge did not change,and there was no release of IOM.In addition,DBPs formation and composition analysis also showed that there was no unstable DBPs such as Hans under the condition of 1 mg/L Cl2/mg MLSS,and TCM formation was 91%lower than that under the condition of 10 mg/L Cl2/mg MLSS,and the total toxicity of DBPs was only 8.8%of that under the condition of 50 mg/L Cl2/mg MLSS.D-amino acid pretreatment experiment showed that 30 μm D-Tyrosine pretreatment could make the pure Pseudomonas aeruginosa membrane appear obvious cracking,which was helpful to improve the subsequent chlorine cleaning efficiency.Therefore,high frequency and low chlorine chemical cleaning combined with D-amino Acid Pre cleaning can be used to control the generation of DBPs and reduce the health threat and environmental risk in the process of MBR chemical cleaning. |
